Devices may sense the presence (or absence) of particular molecules. For example, a miniature or hand-held spectrometer might be used to detect biological, chemical, and/or gas molecules. Such devices might be useful, for example, in the medical, pharmaceutical, and/or security fields. By way of example, a hand-held device might be provided to detect the presence of explosive materials at an airport.
In some sensing devices, light reflected from a sample of molecules is analyzed to determine whether or not a particular molecule is present. For example, the amount of light reflected at various wavelengths might be measured and compared to a known “signature” of values associated with that molecule. When the reflected light matches the signature, it can be determined that the sample includes that molecule.
In some sensing devices, a Fabry-Perot filter such as the one illustrated in FIG. 1 is used to analyze light reflected from a sample of molecules. The filter 100 includes a first partially reflecting mirror 110 and a second partially reflecting mirror 120 that define a resonant cavity C. Broadband light enters the filter 100, and some photons reflect off of the first mirror 110 while others pass through the mirror 110 and enter the cavity C. While in the cavity C, the photons bounce between the first and second mirrors 110, 120, and eventually some of the photons pass through the second mirror 120 and exit the filter 100.
As the photons bounce within the cavity C, interference occurs and an interference pattern is produced in light exiting the filter 100. As a result, light having a specific wavelength may exit the filter 100. Note that the interference occurring within the cavity C is associated with the distance d between the two mirrors 110, 120. Thus, the filter 100 may be “tuned” to output a particular wavelength of light by varying the distance d between the mirrors 110, 120 (e.g., by moving at least one of the mirrors 110, 120).
In some cases, one of the mirrors is formed using a diaphragm that can be flexed to change the distance d. For example, FIG. 2 is a side view of a Fabry-Perot filter 200 implemented using a flexible diaphragm mirror 210 and a fixed mirror 220. By measuring light reflected from a sample using various distances d (i.e., at various wavelengths), and comparing the results with a known signature of values, it may be determined whether or not a particular molecule is present in a sample. The diaphragm 210 might be flexed, for example, by applying a voltage difference between the mirrors 210, 220.
Such an approach, however, may have disadvantages. For example, the curving of the flexible diaphragm mirror 210 may limit its usefulness as a Fabry-Perot mirror. Moreover, the use of a flexible diaphragm mirror 210 may introduce stress over time and lead to failures. The design might also require bonding materials together that have different thermal characteristics—which can lead to problems at relatively high, low, or dynamic temperature environments. In addition, as the size of the cavity C is reduced, it can be difficult to efficiently control the movement of the flexible diaphragm mirror 210. Note that the use of piezoelectric elements to move mirrors arranged as in FIG. 2 can result in similar problems.